1. Field of the Invention
The present invention relates to a speech correction apparatus for correcting a signal level of guidance speech or the like produced from a car-mounted navigation system in accordance with ambient noise.
2. Description of the Related Art
In general, guidance speech produced from a car-mounted navigation system is hard to hear depending on sound pressure levels of sounds generated from a car audio system or engine sound. The guidance speech can be heard more easily by reducing the volume of the audio system when the guidance speech is provided. In this method, however, the volume of sound from the audio system must be reduced every time the guidance speech is frequently produced, and the user will find it inconvenient. Further, such measures cannot be applied for an uncontrollable sound source, such as engine sound. The user will find it convenient if he/she can understand the guidance speech without reducing the volume of sound from the audio system. In order to satisfy such a requirement, a speech correction apparatus using a loudness compensating technique based on an auditory sense theory has been known (for example, see Japanese Unexamined Patent Application Publication No. 11-166835 (pp. 3 to 7 and FIGS. 1 to 10)). In this speech correction apparatus, a microphone is set at a hearing position and the gain of output speech is corrected according to the noise level at the hearing position. Accordingly, the user can clearly understand the speech regardless of the noise level.
In the speech correction apparatus disclosed in the above-identified Patent Document, the sound pressure level of each of guidance speech produced from a navigation system and ambient noise such as sounds from an audio system is detected so as to determine the gain for amplifying the guidance speech. The sound pressure level is actually detected by averaging the power of guidance speech and ambient noise at the time period just before determining the gain over a predetermined time period. Therefore, at the head portion of the guidance speech, the sound pressure level of the guidance speech is averaged over the predetermined time period, the sound pressure level is wrongly detected at a lower value than the actual value, and thus the sound pressure level at the head portion is excessively corrected disadvantageously.
FIGS. 15A and 15B show an example of excessive correction of a speech head. FIG. 15A shows a waveform of guidance speech to be corrected. FIG. 15B shows a waveform of guidance speech whose gain has been corrected by using the known speech correction apparatus disclosed in the above-identified Patent Document. As can be seen in the figures, the head portion of the guidance speech is excessively corrected and a pulse-like waveform appears.
In the speech correction apparatus disclosed in the Patent Document, guidance speech at the microphone position is estimated by using an adaptive filter. Usually, each filter coefficient of the adaptive filter is set at 0 in the initial state just after power-on, and then the filter coefficient is updated. In the initial state or just after the initial state, the power of guidance speech estimated by using the adaptive filter is much smaller than the actual value. Accordingly, the sound pressure level of the head portion corresponding to the filter coefficient in the initial state is excessively corrected disadvantageously.